This invention relates to high voltage generation systems for semiconductor memory devices, and more particularly to systems for providing high voltages required to program memory devices.
Non-volatile memory devices, such as electrically erasable programmable read-only memories (EEPROM), including flash EEPROMs memories, are widely used. Flash memories comprise memory cell transistors, each have a floating gate, a control gate, a source and a drain. As such, each memory cell transistor retains binary information representing a charge injection state of its floating gate. For example, electrically charging the floating gate of a memory cell transistor brings a threshold voltage of that memory cell into a high state. When the threshold voltage is raised relative to the control gate, the memory cell prevents a current from flowing. Electrically discharging the floating gate of the memory cell lowers the threshold voltage with respect to the control gate, which allows the current to flow through the memory cell. Illustratively, bringing the threshold voltage of the memory cell higher than a word line selecting voltage level of a read state is called an erasure operation. On the other hand, bringing the threshold voltage of the memory cell lower than the word line selecting voltage level of the read state is called a programming operation. Alternatively, the erasure state and the programming state may be defined inversely in terms of threshold voltage.
In flash memory operation, higher voltage level than external power supply is used for erase operation, and lower voltage level than external power supply is used for program operation. Also, in flash memory operation, the voltage level needs to be monitored. Conventionally, a high voltage pad is needed for monitoring the high voltage level.
In a conventional system for generating high voltage for programming and erasing data from flash memory device, it includes a current reference, a current controlled oscillator, a clock generator, a charge pumping circuit, a high voltage regulator and feedback circuit. Such systems have complicated circuit designs that also require large areas on the chip. Therefore, conventional systems are not suitable for low cost non-volatile memory applications.
Desirable in the art are additional designs of a high voltage generation systems that provide simpler regulating circuits for regulating program and erase voltages, and monitoring the high voltage level thereof.